Wireless communication systems are commonly used by public service organizations, such as police, fire, and ambulance squads, and by private organizations, such as trucking companies, to provide a means to communicate with each other. Recently, developments in the wireless communication field have allowed systems to be created that can process both voice and data transmissions. An example of such a system is the OpenSky® System developed by M/A-COM, Inc. (Lowell, Mass.).
Systems in use today, such as OpenSky®, generally comprise at least one base station that communicates with one or more individual radios, consisting of mobile units or portable units. “Mobile units” are generally understood to be units that are located in a vehicle, while “portable units” are generally understood to be units that are hand carried by a user. Each base station is configured to transmit high power RF energy from an antenna, normally located at a significant height above ground. Each base station is also capable of receiving RF energy via an antenna. The base station communicates with individual radio units that are close enough geographically to transmit and receive an RF signal of a minimum required power level for successful communication.
Communication systems often comprise more than one base station. Each base station is linked to a central office using a land based or wireless system designed not to affect the communication system on which the base station operates. The central office performs various switching and routing functions that allow several base stations to operate together as a single wireless communication system.
In addition to one or more base stations, a plurality of mobile units and portable units can operate on a given system. Mobile units typically have more range than portable units, meaning that they can maintain effective communication at greater distances from the base station. The reason for this is that mobile units are fixed within a vehicle, thus, a battery carried by the vehicle powers them. A typical mobile unit can transmit RF signal at a power level of approximately 20 Watts. Much smaller batteries built into the radio devices, however, power portable units. As a result, they are limited in the amount of signal power at which they can transmit. A typical portable unit transmits RF signals at a power level of approximately 3 Watts. Thus, the range of a portable unit is significantly less than the range of a mobile unit.
In addition, portable units are often further limited by the user location. For example, a user can carry a portable unit into the basement of a building or other structure, which has the effect of further limiting the range of the unit. As a result, there are often areas where communication between the base station and the portable unit is degraded or lost completely.
One method that has been used to improve the communication range of portable units is using mobile repeater units. A mobile repeater unit is a unit that is permanently mounted in a vehicle that acts as a communications relay between the portable unit and the communication system As such, the mobile repeater unit acts as an intermediary between portable units and the base station. It receives information from the portable units over a “repeater channel” and forwards the information to the communication system over a “base channel” for a particular base station that is part of the communication system. Similarly, it receives information from the communication system via the base channel and relays it to the portable units over the repeater channel. By performing this function, mobile repeater units improve the coverage area in which portable units can operate efficiently. Portable units that are proximate to a mobile repeater unit but in an area beyond where their signal can reach the base station directly can communicate to the base station via relayed transmissions through the mobile repeater unit.
The mobile repeater unit is typically combined with a mobile radio unit, so a typical vehicle mounted mobile repeater unit allows a user in the vehicle to communicate directly with the base station while also functioning to relay communications from portable units located nearby. For the purposes of this discussion, the term “mobile repeater unit” shall hereinafter refer to mobile vehicle repeater units with mobile radio capabilities. Co-pending application Ser. No. 09/696,930 filed Oct. 25, 2000 further describes mobile repeater units operating within a wireless communication system, and is incorporated herein by reference.
One problem that exists on systems using mobile repeater units is that there is a need for the portable units and mobile repeater units to communicate on a common frequency or channel. Typically, the mobile repeater units and portable units will both be in possession of a list of repeater channels, but will not have any priority established about on which repeater channel to operate. Moreover, because mobile repeater units are by nature mobile, it is possible, and in fact probable, that more than one mobile repeater unit could be located within the transmission range of a portable unit. Since multiple mobile repeater units operating on the same channel in close proximity to each other could interfere with each other, and since repeater channels are a scarce resource that must be shared between a plurality of mobile repeaters and their associated portable units, it is desirable to assign the repeater channels in a manner that prevents interference and makes efficient use of the repeater channels. It is also desirable that a portable unit communicate with the mobile repeater unit that provides the best service in accordance with some useful criteria.
Furthermore, since the mobile repeater unit may be unattended and the user of the portable unit may be fully engaged in his primary mission, it is desirable that the mobile repeater unit and the portable unit establish communication automatically without any user input.
Therefore, it is desirable to have a means for a portable unit to automatically establish communication with one of a plurality of mobile repeater units and in so doing identify an operating channel that prevents other proximate mobile repeater units from operating on the same channel, while also providing the best service to the portable unit and making efficient use of available channel resources. The present invention fulfills these needs, among others.